A telecommunications network consists of a number of switches and application processors interconnected by transmission circuits; in traditional networks, signalling systems use dial pulse and multiple frequency tones to transmit calls and circuit related information. These signalling systems lack the capability to handle additional services to the user. It was therefore suggested to provide a separate signalling network in addition to the traditional network used for routing communications. Common Channel Signalling was devised for such a signalling network. It is a signalling method, in which a sigalling channel conveys signalling information by means of labelled messages; examples of common channel signalling systems are CCITT Signalling System no. 7, and various national versions such as the Bell Communications Research (Bellcore) and AT&T SS7 standards. SS7 architecture is detailed in these standards, or in SS7 Basics of Toni Beninger, Telephony, Div. Intertec Publishing Corp., Sept. 1991. SS7 is designed to provide an internationally standardised, general-purpose signalling system. The invention is discussed in the rest of the specification in the preferred embodiment of SS7 protocol.
SS7—as an example of Common Channel Signaling protocol—is comprised of several layers. It comprises three MTP (Message Transfer Part) levels entitled MTP Level 1 to MTP Level 3. MTP level 1 corresponds to the OSI layer 1 (physical layer); MTP Level 2 corresponds to OSI layer 2 (Data Link layer), and MTP level 3 corresponds to the bottom of OSI layer 3 (Network layer). SS7 also comprises a Signalling Connection Control Part (SCCP) corresponding to the top of OSI layer 3. Further layers are provided, but their description is not necessary for the understanding of the present invention.
MTP Level 1.
MTP Level 1 is known as the signalling data link. A signalling data link is a bi-directional transmission path for signalling, comprised of two data channels operating together in opposite directions at the same data rate.
For operating properly under MTP Level 1, a given point code should know the type of MTP Level 1 protocol used on the SS7 network, as well as the different parameters of this protocol, such as clock signal configuration, bit encoding type, used cable instance.
MTP Level 2.
MTP Level 2, together with MTP Level 1, provides a signalling link for reliable transfer of signalling messages between two directly connected signalling points. Message formats comprise three types of signal units: message signal units (MSU), link status signal units (LSSU) and fill-in signal units (FISU). Fill-in signal units are normally transmitted when not MSUs or LSSUs are being transmitted; this allows the SS7 network to receive immediate notification of signalling link failure.
In addition to defining these three types of messages, MTP Level 2 provides an initial alignment procedures (IAP); these procedures occur when a signalling link is activated for the first time or restored after a link failure. There is provided a normal alignment procedure, and an emergency alignment procedure. A “proving state” is provided included in both procedures to measure error rates, ensuring that a reliable link is established. A SS7 point code issuing a request for alignment under the normal IAP expects to receive an answer within a time predetermined by a timer.
For operating properly under MTP Level 2, a given point code should know the time slot in case a time division multiplex is used.
MTP Level 3.
MTP Level 3 provides the functions and procedures related to message routing and network management. MTP Level 3 handles these functions assuming that signalling points are connected with signalling links are described in MTP Level 1 and MTP Level 2. More specifically, after MTP Level 2 alignment is carried out, a SS7 point code issues a signalling link test message (SLTM).
One problem with SS7 devices is that they need to be configured for proper operation within an existing SS7 network. This requires a large SS7 protocol knowledge, a complete documentation and some time. This problem appears not only when a new SS7 platform is installed, but also whenever a new SS7 device is added to an existing netwrok, or modified in an existing network. In other words, apart from the installation problem, there is a problem in dynamically configuring a SS7 platform.